The present invention relates to a non-volatile memory device and a method of programming a multi-level cell. More particularly, the present invention relates to a page buffer for efficiently programming a most significant bit in a multi-level cell.
Recently, the demand for a non-volatile memory device which programs/erases data electrically and does not need to have the data periodically refreshed has been increased. In addition, to enhance the degree of integration of the non-volatile memory device, studies concerning a cell for storing data above one bit have been actively pursued.
Hereinafter, a non-volatile memory device having a cell for storing data with a plurality of bits is referred to as a multi level cell (MLC) non-volatile memory device.
For example, in the non-volatile memory device having the multi-level cell for storing data of 2 bits, one cell can store one of the four possible data combinations, i.e. 11, 10, 01 and 00. As a result, the degree of integration of the non-volatile memory device may be increased in view of the increased logic.
A program operation about cells in the MLC non-volatile memory device includes an operation for programming a cell in steps unlike a single level cell (SLC) non-volatile memory device having a single level cell for storing one bit. Additionally, the MLC non-volatile memory device may store data having a plurality of conditions through the above operation.
In this case, a part of cells in the MLC non-volatile memory device are first programmed when a program operation about specific cells included in one page is performed.
Other cells are once programmed in a previous step. Here, when a program operation about the cells programmed once is again performed, a phenomenon that threshold voltages of the cells are not increased may occur if a program voltage of above a given value is not applied to the cells. As a result, a time required for programming cells already programmed once is increased compared with that required for the first programming, and so a program disturbance phenomenon may occur.